i think that Blender supports a number of types of plugins to allow additional control over making.

Old Blender (2.49) had texture and sequencer plugins. Those do not work in New Blender (2.6x). New Blender has a completely different system of add-ons and provides access to a lot of internal data via Python scripting.

Don't know about muscle add-ons, although the topic has been kicked around for a while.